Ink jet printers are non-contact printers in which droplets of ink are ejected from one or more nozzle orifices so as progressively to build up a printed image on a substrate moved relative to the nozzle. One form of ink jet printer comprises a source of ink, typically a reservoir or bottle of ink, which is pressurised to from 0.1 to 2 bar, notably about 1 bar. The pressure is created, for example, by pressurising the air space above the ink in the bottle or reservoir. The ink is fed to the nozzle orifice(s) in a print head through which it is ejected as a series of droplets onto the surface of the substrate. The flow of ink through each nozzle orifice is controlled by a solenoid valve. Typically, such a valve comprises an electromagnetic plunger journalled for axial movement within an axially extending electric coil. The distal end of the plunger is located within a valve head chamber through which ink flows from the reservoir to the nozzle orifice. When current is fed through the coil, this generates a magnetic field which acts on the plunger to move it axially and thus open, or shut, the inlet of a bore from the valve head chamber to the nozzle orifice. Typically, the magnetic field acts to retract the plunger against the bias of a coil spring to create a flow path between the valve head chamber and the nozzle orifice. When the electric current no longer flows in the coil, the magnetic field ceases and the plunger returns under the bias of the spring to close the flow path to the nozzle orifice. Typically, a plurality of nozzle orifices are formed as one or more rows in a plate, the nozzle plate, and each nozzle orifice is served by a separate solenoid valve, so that droplets of ink can be ejected independently from one or more of the nozzle orifices. Typically, the valves are fed with ink from the reservoir via a manifold which serves to split and even the ink flow between each of the valves. The row of nozzle orifices is typically aligned transversely to the direction of travel of the substrate so that simultaneous operation of the valves will cause a row of ink dots to be printed on the substrate.
The valves are operated so as to deposit dots upon the substrate at the desired locations on the substrate to build up the elements of a five, seven, eight or more dot raster image on the substrate. By suitable timing of the opening of the various valves in the print head an alphanumeric or other image can be formed on the substrate to print a date, product batch code, logo, bar code or other image on the substrate. If desired, several print heads can be combined in an array so as to print a wider image on the substrate and the line of nozzle orifices in the print head can be angled to the direction of movement of the substrate so as to lay down droplets which are more closely spaced than where the print head is aligned normal to the line of travel of the substrate.
For convenience, the term drop on demand printer will be used to denote in general such types of ink jet printer.
The size of the printed dot can readily be altered by varying the duration for which the valve is held open, and hence the amount of ink that it allows to flow through the nozzle orifice. The form of the image which is printed can readily be altered by varying the sequence of operation of the valves in the print head so that droplets are ejected from the appropriate nozzles in the appropriate sequence to form the desired image. Such alterations of the images and the dot sizes can readily be controlled by a computer or microprocessor operating under an appropriate program or operating system. Such drop on demand printers are widely available commercially and find widespread use in printing a wide range of both visible and non-visible machine-readable images on a wide range of substrates.
However, as the speed of travel of the substrate past the print head increases, a point is reached at which the valve cannot be operated at sufficient speed to eject droplets at sufficient frequency to form the desired image without creating some distortion. Typically, the limit for the speed of operation of solenoid valves in current use in an ink jet printer head is less that 800 to 1000 Hz. With increasing pressure on manufacturers to increase through put from a given production or packaging line, there is an increasing need to be able to print the dots onto the substrate at rates greater than this.
In an alternative form of ink jet printer known as an impulse jet printer, a piezoelectric crystal or other transducer is applied to or forms part of a wall of an ink jet chamber having an ink inlet and an ink outlet to a nozzle orifice. When a voltage is applied to the transducer, the transducer expands or flexes and causes a change in the volume of the ink jet chamber. This causes a droplet of ink to be ejected from the chamber and to exit through the nozzle orifice. The transducer can be caused to flex at very high rates by electronic control of the frequency of the electrical pulses applied to the transducer, so that such a print head can apply dots at frequencies up to 15 kHz or more. However, the volume of ink ejected through the nozzle orifice is dependent upon the extent of flexing of the transducer. This can be varied by varying the amplitude of the electric pulse applied to the transducer. However, each type of transducer operates consistently only within a narrow percentage, typically plus or minus 50%, of the optimum operating pulse amplitude, so that only a limited range of dot sizes can be achieved with a commercially available impulse jet printer. This limits the number of applications for which a given impulse jet head can be used for.
It has been proposed in International Patent Application No PCT/SE97/01007 to produce a solenoid type valve for a drop on demand ink jet printer which is claimed to be capable of operating at frequencies of up to 3 kHz. Such a valve incorporates light weight components so as to reduce the inertia of the plunger and thus enable it to accelerate and decelerate rapidly at each extreme of its travel within the coil. To achieve this, the plunger is formed from two components, one made from an electromagnetic material so that it can be caused to move by the magnetic field generated by the current passing through the coil, and a second lightweight plastic component for the distal end of the plunger. Such a construction is complex and expensive. Furthermore, we have found that a print head incorporating such a valve design does not print acceptable images. For example, at high frequencies of operation of the valve, the printed dots are uneven and there are many small satellite dots around each of the primary dots printed by the print head.
We have now devised a form of valve which can be operated at speeds of up to 8 kHz or more and yet can be used in a drop on demand printer to print uniformly sized droplets over a surprisingly wide range of dot sizes and operating frequencies. Furthermore, the valve of the invention can be more compact and with smaller components than a conventional design of solenoid valve for use in a drop on demand ink jet printer. This allows high definition printing to be achieved at high print rates without excessive heat being generated during operation of the valve. Such a valve enables drop on demand technology to be used in high speed applications for which an impulse jet print head had hitherto been considered the only technically viable form of print head.